This invention relates to a moisture vapor permeable membrane and, more particularly, to a reinforcing of the membrane for dimensional stability which renders the membrane suitable for construction of a surgical gown or drape, a covering for a wall of a house, as well as a covering of a container for industrial processes.
Moisture vapor permeable membranes in the form of films may be made from synthetic polymers and formed by casting, extrusion or other known film-making processes. Film thickness is in a range of typically 0.5-10 mils and preferably in a range from 0.6-3 mils. The films are continuous and are formed generally of hydrophilic polymeric materials through which water vapor is capable of diffusing. Such films may be of a plastic material such as copolyether polyester, or polyurethane or acrylate copolymers as disclosed in McCracken et al, U.S. Pat. No. 4,413,621. One form of such film is referred to as a monolithic film, and has no holes produced therein by a physical processing of the film but, rather, has passages with cross-sectional sizes on a molecular scale formed by a polymerization process and serving as conduits by which water molecules can propagate through the film. A second form of the film is known as a microporous film, and has microscopic holes produced therein by a physical stretching of the film provided during construction of the film. In certain applications wherein increased flow of fluid through the film is required, an array of holes punched by fine needles may be provided within the film; however, the cross-sectional dimensions of such holes are larger by many orders of magnitude than the passages of microporous and monolithic films. Generally, the microporous and monolithic films have moisture vapor transfer rates between 15 and 80 grams per 100 square inches per 24 hour interval at a temperature of 100.degree. Fahrenheit and 90% Relative Humidity.
By way of example in the use of such films, benefits of the film have been demonstrated by use of the film as a surgical dressing as disclosed in Martz, U.S. Pat. No. 4,846,164. The surgical dressing may be constructed as a laminate including a layer of the film with a gauze pad for absorption of exudate from a patient, and wherein the laminate may include also some form of backing layer to facilitate exposure of an adhesive surface for emplacement of the dressing on a wound. The film is impermeable to liquid water and to bacteria so as to form a very effective shield which protects a patient from sources of infection external to the skin. The film retains body fluids within the body at the site of the wound. The vapor permeability of the film provides a sufficient rate of water vapor transport through the film to allow the skin to breathe normally, and is useful, therefore, not only in the construction of dressings, but also in the construction of garments wherein the characteristic of vapor permeability gives a cool feel rather than a sensation of excess heat. The film has sufficient elasticity to conform to the shape of various parts of the body, even a flexible body part such as a knee or elbow. Both the film and the adhesive layer may be constructed to be transparent or opaque as may be desired for particular applications of the film. Materials used in the construction of the film are non-allergenic.
The foregoing benefits in the use of the membrane are not restricted to surgical dressings, but can be employed to advantage in the construction of a much larger covering, such as a surgical drape or gown which may be large enough to cover part or all of a person. For example, a physician wearing a surgical gown made of a membrane, such as the foregoing film, would experience comfort provided by the breathability of the film, and would be protected from bacterial and even viral infection because of the capacity of the film to shield a person from such sources of infection. The flexibility of the film would allow the physician to move about freely, as is required in the performance of surgical procedures, by way of example. Thus, a film or membrane having the foregoing properties of being breathable and impervious to liquid water would offer protection from liquids, particularly body fluids such as blood which may carry pathogens, microbes, and other extraneous contaminants which may be injurious to the health of a health care worker.
In spite of the many advantages of the thin film type of membrane, there are problems associated with its use. The film is too thin to be handled without some form of backing sheet because the extreme flexibility and limpness allow the film to curl over upon itself. Such difficulty is compounded in the event that an adhesive layer be present on a surface of the film, as might be done to provide for lamination of the film in a manufacturing process. Furthermore, the film is fragile and may catch readily upon a sharp or rough object resulting in a tearing of the film and a loss of the integrity of the film as a barrier to pathogens. Even if some sort of permanent backing layer be applied to the membrane for increased stiffness and resistance to abrasion, then a further problem is introduced, namely, that such backing layer may materially alter the vapor transport rate of the membrane and, depending on the nature of the backing layer, may not allow for any vapor transport. Thus, there are significant disadvantages in the use of such a thin-film membrane for use by health care workers, as well as disadvantages in the handling of such membrane for fabricating products incorporating the membrane.
A further aspect in the use of such membrane or film may be appreciated from a standardized textile test similar to the Mullens test, for example, a standardized test such as ASTM, ES 21 and 22 for measurement of potential penetration by blood which may contain pathogenic microorganisms through a membrane under conditions wherein a hydraulic pressure is applied across a surface of the membrane. Under a pressure of two pounds per square inch, excessive bowing of the film is noted in a test cell diameter of only three inches. The bowing is associated with cold flow, elongation and creep of the plastic breathable material, and produces such a distension of the film that there is danger of rupture as well as, possibly, a change in the physical parameters of the film. For example, any tear produced by the distension would permit a flow of liquid through the film, thus negating resistance of the film to passage of liquid water and blood borne pathogens. In order to prevent such failure to the film under test, the test procedure allows for the use of a metal or plastic grid to support the film against the hydrostatic pressure, and thereby prevent the bowing.
The foregoing bowing and impairment of the film under hydrostatic pressure can appear in other cases of unsupported film as may be demonstrated for various situations in the wearing of garments. For example, a surgeon having his arm covered by such a film may accidentally press his arm against a blood covered surface of a table or other fixed object. Significant hydrostatic pressure builds up between the film and the surface of the blood. This may cause sufficient local distension of the film to disrupt the film resistance to a passage of blood through the film. As a further example, a ski glove constructed of a laminated fabric which includes an inner layer of breathable film may be worn by a skier who is squeezing a snow covered ski pole. Unless the film is supported by a layer of fabric, the hydrostatic pressure of melted snow against the film might cause excessive distension and rupture of the film, and the skier's hand would get wet. A similar result is obtained in the situation wherein the skier is wearing a laminated ski suit, one of the inner layers being a breathable film or membrane. If the skier sits on an ice covered seat of a ski lift, water of melted ice would pass through the outer layers of the garment and create significant hydrostatic pressure against the film. If the film is unsupported by other layers of the garment, there may be a resulting bowing of the film which impairs resistance of the film to liquid transport, and the skier gets wet.
One approach to overcoming the foregoing problems for use of the plastic film material in a surgical gown or drape is disclosed in Molde, U.S. Pat. No. 4,433,026 which discloses (column 3) a cloth-like material comprising a three-layer flexible laminate having a middle layer of plastic film material and two outer layers of plastic fabric materials secured to opposite sides of the middle layer by suitable adhesive means. The middle layer is substantially waterproof and air breathable while one of the outer layers is substantially dimensionally stable and the other outer layer is substantially dimensionally unstable to permit conformance of the fabric to the body of a person wearing the gown made from the fabric. Both the stable layer and the unstable layer are formed from polyester continuous filament yarn wherein, in the dimensionally stable layer, the yarn is woven in a poplin or regular broadcloth weave, while in the dimensionally unstable layer, the yarn is produced as a knit, such as a tricot knit. The resulting gown is reusable, and is rendered free of bacteria after each use cycle by a steam autoclaving of the gown. First, it is clear from the description in Molde, that the Molde gown is a relatively heavy and permanent gown, not intended for throw-away after a single use. A further example of a laminated moisture permeable sheet is given by Watabe, U.S. Pat. No. 4,507,356, wherein a woven or knitted fabric of nylon or polyester, such as nylon stocking material or gauze, serves as reinforcing material for plural layers of moisture-permeable plastic material. In the Watabe laminate, the gauze is treated with an epoxy resin to provide a base for the anchoring of hair in a wig.
However, in many hospitals, it is the practice to employ surgical gowns only once, and after their use to discard the gowns. In such application, is desirable to fabricate the gowns of a thin light-weight fabric to facilitate storage and transport of the gowns. Also, in the use of throw-away gowns, it is desirable to minimize the cost of the gowns. The use of multiple-layer fabrics in conjunction with a breathable film or membrane increases the cost significantly over that of the membrane itself. For the foregoing reasons, the laminated material of Molde would be contraindicated in a hospital procedure requiring a single use of surgical gowns followed by their discard. Furthermore, the use of the woven stocking material or gauze of Watabe provides very fine spacing between the yarns which, in combination with an applied coating of an epoxy or adhesive for attachment to the film, provides a significant reduction in the breathability of the resulting laminate as compared to a breathability of such a film without the woven material.
It is noted also that the foregoing use of the breathable film or membrane in the surgical gown or other garments is provided by way of example, and that there are other uses of such membranes such as in construction and industrial processes wherein a minimization of weight, thickness, and cost, and improved resistance to hydraulic pressure are advantageous.